Liquid recovery from an originally vaporous mixture



A ril 3, 1962 LIQUID RECOVERY FROM AN ORIGINALLY VAPOROUS MIXTURE FiledJune 15, 1959 H. FORBES ETAL INVENTORS:

GODFRIED J.VAN DEN BERG HENRY FORBES HUBRECHT VAN DER MAREL THEIRATTORNEY Sfiltes Patent Patented Apr. 3, 1962 3,028,332 LIQUID RECOVERYFROM AN ORIGINALLY VAPOROUS MIXTURE Henry Forbes, Huhrecht van derMarc], and Godfried J. van den Berg, The Hague, Netherlands, assignorsto Shell i] Company, New York, N.Y., a corporation of Delaware 7 FiledJune 15, 1959, er. No. 820,517 Claims priority, application NetherlandsOct. 7, 1958 9 Claims. (Cl. 208-340) This invention provides animprovement in the process involving pressurization of an originallyvaporous hydrocarbon mixture, containing normally gaseous components, torecover further liquid therefrom, which improvement permits asignificant reduction in the total compression work required for theoperation.

It is conventional practice in the processing of a vaporous hydrocarbonstream, containing normally gaseous components, to pressurize the streamto increase liquid recovery therefrom. The stream may be first cooled tobring about a condensation, following which the stream is separated intoa gaseous portion and a liquid portion. The individual portions are thenseparately pressurized to substantially the same elevated pressure andthen recombined for further processing. There is an economic advantagein separately pressurizing the two portions. Processing schemes of thisgeneral nature are used in refinery gas recovery and for gasolinestabilization in some instances. A process of this general type isdescribed in Petroleum Refiner, page 232, September 1949.

It is an object of the invention to provide an improvement in theprocess of pressurizing an original vaporous hydrocarbon mixture andmore particularly to provide an improved process reducing the amount ofwork required in pressurization. This and other objects will become moreapparent in the following description of the process, taken inconjunction with the drawing which is a schematic representation of apreferred embodiment of the improved process.

It has now been discovered that in the processing of a vaporoushydrocarbon stream, containing normally gaseous components, to increasethe liquid recovery therefrom, that it is advantageous to recycle aportion of the recovered liquid to the vaporous hydrocarbon streamundergoing treatment. In the improved process the stream is cooled tobring about a condensation, following which it is separated into agaseous portion and a liquid portion and the individual portions arethen separately pressurized to substantially the same elevated pressure.Following pressurization, light components are conjointly liberated fromthe two pressurized portions to obtain a liquid fraction. A portion ofthis liquid fraction is returned to the vaporous hydrocarbon stream at apoint preceding its separation into the gaseous and liquid portions. Therecycling of the portion of the recovered liquid in this manner willsignificantly reduce the total compression work required for thepressurization of the two separate portions.

The term pressurizing and the like is employed to mean raising thepressure to which the vaporous mixture is under in the initial stage ofthe process which original pressure may be atmospheric,superatmospheric, or subatmospheric. The recycled or returned portion ofthe recovered liquid may be a portion of the first liquid initiallyseparated or a fraction from that liquid obtained say in a laterdistillation.

The conjoint liberation of the relatively light components from thepressurized gaseous and liquid portions may, for instance, be broughtabout by treating the two portions together (premix or otherwise) in adistillation column with the light components being removed over head.In this instance the gaseous portion may, if desired, be introduced intothe distillation column on a tray which is higher than the one on whichthe liquid portion is introduced. It is, however, often advisable to mixthe two pressurized portions and then to liberate the light componentstherefrom by subsequent cooling of the resultant mixture to promotecondensation, after which the mixture is passed to a separator whereinit is separated into gas and liquid. The separated liquid is preferablyfurther treated in a distillation to remove additional light componentstherefrom, yielding a heavy liquid fraction which may be combined withthe original vaporous mixture, or it may be deemed more feasible to passthe heavy liquid fraction to another fractionation to obtain a stillmore heavier material for the recycling.

Preferably, the recycled liquid portion is added to the vaporoushydrocarbon stream undergoing treatment at a point preceding the coolingof that stream, although it may be combined with the cooled streambefore its separation into its vaporous and liquid portion and prior totheir separate pressurizations.

The vaporous stream undergoing treatment may be the top product of aprimary distillation operated at approximately atmospheric pressure. Thefeed to the primary distillation column may, for instance, be ahydrocarbon mixture relatively rich in light components (such ashydrogen and methane) which is therefore difiicult to condense; suchhydrocarbon mixtures include the effluent from a reforming unit or froma hydrodesulfurization treatment.

In a preferred embodiment of the process the top product will be agasoline range material containing lighter components from anatmospheric primary distillation. This top product prior to cooling iscombined with a recycle heavy gasoline (naphtha), then cooled and passedto a separator maintained say at 1.1 atmospheres and around 45 C. Therethe liquid and gas phases separate and are separately removed,pressurized and subsequently recombined, passed to a second cooler toobtain further condensation, and then to a second separator maintainedat say, 5 atmospheres and 40 C. A gaseous stream containing principallynon-condensibles is removed from this latter separator. The liquidrecovered in this second separator is moved to a secondary distillation(a dcbutanizer) where the butane and remaining lighter components aredistilled overhead. This column may operate at around 12 atmospheres.The liquid gasoline product from the base of the debutanizer goes to afractionation column where it is separated into a top product boilingbelow say 93 C. and a bottom product having the boiling range ofapproximately 93 C. to C. A portion of this bottom product is nowrecycled according to the invention to combine with the top product fromthe primary distillation column at a point preferably preceding itsinitial partial condensation. As a result of the recycle the compressionwork to be exerted on the liquid portion is admittedly slightlyincreased, but that on the gaseous portion is greatly reduced. Sincefrom an econornic point of view it is much more attractive to pressurizea liquid instead of a gas, the practice of the process of the inventionresults in a considerable reduction of the total compression workrequired in the pressurization of the two portions.

When it is desired to change the cutting point in the primary column(for instance, when a change is necessary to the preparation of aproduct answering difierent specifications), the quantity of therecycled liquid fraction which is combined with the primary top productshould be accordingly varied to hold the gas-compression worksubstantially constant, thus permitting the gas compressor to operate(with changes in the primary distillation cutting points) at a capacityapproaching the optimum. For instance, if the cutting point in theprimary column is varied to take overhead a larger proportion of heaviermaterial, the amount of recycle required will be less. It is recommendedthat the amount of recycle material em ployed be such that the totalquantity of debutanized gasoline removed from the secondary distillationcolumn remains substantially constant with variations in the cuttingpoint of the primary column. In the instance where the debutanizedgasoline is passed to a subsequent fractionation where it is separatedinto a light gasoline and a heavy gasoline product, and a portion of theheavy gasoline is used as the recycle material, the quantity of therecycle and the quantity of heavy gasoline actually withdrawn as productshould give a total that remains substantially constant at difierentprimary cutting points. This practice will result, it will be seen, inthe amount of debutanized gasoline being maintained at substantiallyconstant volume.

The invention will nowbe further explained with reference to theaccompanying drawing, which relates to the use of the process of theinvention in working up a hydrocarbon oil having a relatively highcontent of normally gaseous components. The term gases as used hereinincludes vapors. The hydrocarbon stream is introduced through a line 10to an atmospheric primary distillation column '12 which is operated witha cutting point at 165 C. to separate overhead a light fractioncontaining .gasoline and lighter components and a heavier bottomfraction composed of kerosene and gas oil. The heavy fraction is removedfrom the distillation column through a line 14. In the present instancecirculating reflux is supplied to the column via a line 60 opening intothe top of the column. Conventional refluxing may be-employed. The lightoverhead fraction is removed from the top of the column through a line13 to a partial condenser 15 and then to an accumulator 16. Theaccumulator is operated under a pressure of 1.1 atmospheres absolutewith a temperature of approximately 45 C. In the cooler 15 there occursonly a partial condensation, with the result that a partly gaseous andpartly liquid product is collected in the accumulator. A liquid productis removed from the bottom of the accumulator through a line 17, whilethe gaseous product is led through a line 18 to a gas compressor 19. Theliquid flowing in line 17 is compressed by a pump 21 to substantiallythe same pressure as the pressurized gas leaving the compressor 19 inline 20. The two pressurized streams combine in a line 22 which opensinto a cooler 23. From the cooler the combined stream passes to anaccumulator 2.4 which operates at a pressure of approximatelyatmospheres and a temperature of say 40 C. In the accumulator a partlygaseous and a partly liquid product is again collected. Completecondensation of the top product of the primary distillation column 12 isimpossible even at this relatively high pressure of 5 atmospheres andrelatively low temperature, because of the relatively high content oflow boiling components in the feed to the primary distillation column12. The uncondensed gases gathering in the head space of the accumulator24 are discharged through a line 25 and a pressure, reducing valve 26from, the

system. The liquid separating out in the accumulator 24 is removed vialine 27 and is forced under the pressure of a pump 28 to a centralsection of a secondary distillation column 29 where it is separated intoa butane-free gasoline and an overhead fraction containing butane andlower boiling components. The secondary distillation column operatesunder a pressure of approximately 12 atmospheres. The heat needed forthe column is supplied by a reboiler 30. The butane-free gasoline bottomproduct is removed from the column in a line 31 to a fractionationcolumn 32 where it is separated into a light gasoline discharge overheadthrough a line 33 and into a heavy gasoline (naphtha) which is withdrawnthrough a line 34. A reboiler 35 supplies the heat required for theoperation of this column.

The top product of the second distillation column 29 is withdrawnthrough a line 37, cooled in a cooler 38 and collected in an accumulator39. The accumulator is operated under a pressure of 12 atmospheresabsolute and at a temperature of approximately 45 C. A portion of theliquid collecting in the accumulator is returned via a line 40 as refluxto the top of the second distillation column. The rest of the liquid isremoved from the accumulator in a line 41 and forced under the pressureof a pump 42 to the central section of a distillation column 43. In thelatter column the material is separated into C; hydrocarbons on the onehand and lower boiling components on the other. The C fractions aredischarged from the column through a bottom line 45 and the top productis removed via an overhead line 46, to a cooler 47 and an accumulator48. In the accumulator a liquid substantially consisting of propaneseparates which is withdrawn through a line 49 and partially returnedthrough a line 50 as reflux to the distillation column 43. The pressurein the distillation column 43 and the accumulator 48 is approximately 24atmospheres absolute. The temperature in the accumulator is about 45 C.

The uncondensed gases gathering in the accumulators 39 and 48 aredischarged through the lines 52 and 53, respectively, which are providedwith the necessary reducing valves.

According to the present invention, a portion of the naphtha fractionwithdrawn through the line 34 is recycled via a line 55 to the topproduct line 13 at a point preceding the cooler 15. The recycled naphthamay be mixed with the top product beyond the cooler 15 at a pointpreerably combined with the top product stream before the cooler.

EXAMPLE A crude oil is separated by distillation into a fraction boilingbelow 350 C. and a fraction boiling above 350 C. The former fraction issubjected to a hydrodesulfurization treatment in which a cobaltoxide-molybdenum oxide-alumina catalyst is used. After cooling thereaction product is subjected to an expansion in stages to separate thebulk of the dissolved gases and vapors. The liquid finally obtained (486tons per 1000 tons of crude oil), in which small quantities of lightcomponents such as hydrogen, hydrogen sulfide and normally gaseoushydrocarbons are still dissolved, is then separated by distillation intoa number of fractions with the use of the plant shown in theabove-described accompanying drawing.

In the primary column 12 (for which circulating reflux is exclusivelyused again) a bottom product boiling above C. is obtained at aboutatmospheric pressure (1.1 atm. abs); the feed components boiling belowthis temperature pass overhead as primary top product (184.15 tons per1000 tons of crude oil) and are led through the line 13 and the cooler15, where they are cooled to 45 C., into the accumulator 16 where thepressure is 1.1 atm. abs.

The liquid and the vapor are pumped, as shown in the drawing, to theaccumulator 23 where the pressure is 5 atm. abs. The temperature in thisvessel is 40 C. The liquid collecting in the accumulator is removed andthen separated in the secondary distillation column 29 at an elevatedpressure into butane-free gasoline on the one hand and butane+lightercomponents on the other. The top product is obtained with the use of aconventional reflux system, as indicated in the drawing. The pressureand temperature, in the reflux accumulator 39 are 12 atm. abs. and 45 C.respectively.

The partly liquefied top product of the column 29 is separated in thecolumn 43 into a bottom product consisting of butanes, and into a morevolatile top product. The top product is again obtained with the use ofa conventional reflux system, the pressure and the temperature in thereflux accumulator 48 being 24 atm. abs. and 45 C. respectively. Thecondensed top product consists substantially of propane and is withdrawnthrough the line 49.

Gases not condensed in the reflux accumulators 24, 39 and 48 aredischarged through the lines 25, 52 and 53. The bottom product of thecolumn 29 is separated in the column 32 into a top product boiling below93 C. and a bottom product boiling between 93 C. and 165 C. which iswithdrawn through the line 34. Part of this bottom product (27.2 tonsper 1000 tons of crude oil) is now recycled according to the inventionthrough the line 55, as a result of which the work to be done by the gascompressor 19 is considerably decreased. This is shown in the table.

Table Top Product Top Product Quantities of Gases and Primary PrimaryVapors to be Compressed Distillation Distillation by Compressor 19Column 12 Column 12 Cutting Point In 185 C. 165 C. 185 C. 165 C. 165 C.

Distillation In Column 12 Recycling 1 None None 27.2

Quantitiesz 1 In tons per 1000 tons of crude oil.

The second data column of the table shows the quantity of top productsupplied by the primary distillation column 12 at the said final boilingpoint of 165 C. The last two data columns of the table show thequantities of gas from this top product which are to be compressed bythe compressor 19, without and with recycling through the line 55 of aquantity of the bottom product from the column 32 (27.2 tons per 1000tons of crude oil). For the purpose of comparison the first data columnof the table shows the quantity of the top product from the primarydistillation column 12 formed when the cutting point in thisdistillation column is 185 C.; the third column shows the quantities ofgas which are to be compressed in this case by' the compressor 19.

When the bottom product of column 32 is not recycled at a cutting pointof 165 C. nor at a cutting point of 185 C., the quantity of gas to becompressed by the compressor 19 is approximately 37.1 tons per 1000 tonsof crude oil in the first instance and approximately 33.7 tons in thesecond. In the first case the load on the compressor 19 is 9.9% greaterthan in the second. When the recycling is, however, effected at aprimary cutting point of C., the quantity of gas to be compressed is notmore than about 34.7 tons per 1000 tons of crude oil, i.e., a quantityonly slightly greater than the quantity to be compressed at a cuttingpoint of C.; the diflerence in compressor load at the two final boilingpoints is now not more than 2.5%. Recycling thus makes it possible touse the same (relatively small) compressor even though the cutting pointin the primary column is altered, thereby improving the operationaleconomy and also the flexibility of the plant.

In this case the recycled quantityof the bottom product of thefractionation column 32 is equal to the difference between the quantityof the bottom product (of column 32) at a primary distillation cuttingpoint of 185' C. (145.4 tons per 1000 tons of crude oil) and thequantity of bottom product at a primary cutting point of 165 C. (118.2tons per 1000 tons of crude oil), there being no recycling in the lattercase. In other words, care is taken to ensure that the total quantity ofbottom product (viz. the quantity to be recycled plus the quantityimmediately withdrawn) is substantially constant at the differentprimary cutting points.

We claim as our invention:

1. In the processing of a vaporous hydrocarbon stream containingnormally gaseous components to increase the liquid recovery therefrom,wherein the stream is cooled to effect a condensation, following whichthe stream is separated into a gaseous portion and a liquid portion andthe individual portions are separately pressurized to substantially thesame elevated pressure, the improvement comprising conjointly liberatinglight components from the two pressurized portions to obtain therefromas one new portion a liquid fraction recovered by fractionaldistillation and returning a portion of said liquid fraction to thehydrocarbon stream at a point preceding its separation into the gaseousand liquid portions, thereby significantly reducing the totalcompression work required for the pressurizations of the two separatedportions.

2. A process in accordance with claim 1 wherein said returned portioncomprises a further heavy fraction obtained from said liquid fraction.

3. A process in accordance with claim 1 wherein the returned portion ofthe liquid fraction is combined with the vaporous hydrocarbon stream ata point preceding the cooling of said stream.

4. A process in accordance with claim 1 wherein a first liberation oflight components is effected by 1) mixing the two pressurized portions,(2) cooling the resulting mixture, and (3) thereafter separating saidlight components from the cooled mixture.

5. A process in accordance with claim 4 wherein the liquid separatedfrom the light components in step (3) of that claim is subsequentlydistilled at a still more elevated pressure to separate further lightcomponents therefrom to provide a heavy liquid fraction, a portion ofwhich is returned to the process at a point preceding the initialseparation of the hydrocarbon stream.

6. A process in accordance with claim 5 wherein the heavy liquidfraction of that claim is subjected to a still further distillation toseparate another heavy liquid fraction which is recycled in part to theprocess at a point preceding the initial separation of the hydrocarbonstream.

7. A process in accordance with claim 4 wherein the vaporous streamundergoing treatment is a reformed naphtha and the light componentseparated in step (3) of claim 4 are principally normallynon-condensibles including hydrogen and the liquid fraction of step (3)is a gasoline fraction containing butane and wherein the liquid fractionis passed to a first distillation to separate the butane and lightcomponents therefrom and the resulting debutanized gasoline fraction issubjected to a further distillation to separate a light gasoline productoverhead from a heavy gasoline product which is returned in part to thevaporous stream of reformed naphtha prior to its cooling and initialseparation.

8. A process in accordance with claim 1 wherein the vaporous hydrocarbonstream is the top product of a primary distillation operating atapproximately atmospheric pressure and wherein said liquid fraction ofclaim 1 is derived from a second distillation and a portion of thatliquid fraction is the material returned to the top product of theprimary distillation.

9. A process in accordance with claim 8 wherein, when the distillationcutting point in the primary distillation is changed, the quantity, ofthe liquid fraction combined with the top product is so regulated inamount that the total quantity of heavy components Withdrawn from thesecondary distillation remains substantially constant.

References Cited in the file of this patent UNITED STATES PATENTS OTHERREFERENCES Fetroleurn Refiner, vol. 28, No. 9, 1949, pages 213, 216,217, 220, 22 1, 224, 225, 229, 232,233, 236, 237 and 240.

Petroleum Refiner, April 1950, pages 97 to 100.

1. IN THE PROCESS OF A VAPOROUS HYDROCARBON STREAM CONTAINING NORMALLY GASEOUS COMPONENTS TO INCREASE THE LIQUID RECOVERY THEREFROM, WHEREIN THE STREAM IS COOLED TO EFFECT A CONDENSATION, FOLOWING WHICH THE STREAM IS SEPARATED INTO A GASEOUS PORTION AND A LIQUID PORTION AND THE INDIVIDUAL PORTIONS ARE SEPARATELY PRESSURIZED TO SUBSTANTIALLY THE SAME ELEVATED PRESSURE, THE IMPROVEMENT COMPRISING CONJOINTLY LIBERATING LIGHT COMPONENTS FROM THE TWO PRESSURIZED PORTIONS TO OBTAIN THEREFROM AS NEW PORTION A LIQUID FRACTION RECOVERED BY FRACTIONAL DISTILLATION AND RETURNING A PORTION OF SAID LIQUID FRACTION TO THE HYDROCARBON STREAM AT A POINT PRECEDING ITS SEPARATION INTO THE GASEOUS AND LIQUID PORTIONS, THEREBY SIGNIFICANTLY REDUCING THE TOTAL COMPRESSION WORK REQUIRED FOR THE PRESSURIZATIONS OF THE TWO SEPARATED PORTIONS. 